I didn't mention anything about alerts, the theory was about how the difficulty of stopping an invasion (based on how far it has progressed) matches the reward. Alerts being generated would be a completely separate mechanic.
What reward, though? Keeping Muruidooges in Invasion for longer would have resulted also in keeping it in Recovery for longer, but again, what is the reward? To assert that this would have slowed the Thargoid advance more so than a quick defence is to assert implicitly that the Alert process would attempt to choose that system a few times but fail and therefore cancel that Alert. Perhaps so, but not necessarily—it might just keep trying more systems until it succeeds in placing the Alert, in which case a slow defence and a quick defence would have equal value/reward.
Incidentally, you did mention Alerts, but rightly so! If the reward is measured in terms of reducing affected systems, the Alerts become connected with that fundamentally, because the change in affected systems is equal to Alert minus Complete.
Their total number of successes wouldn't be affected, so this defence would come at the cost of allowing them freer expansion elsewhere. But depending on what was being walled off from them, that might be considered acceptable.
Indeed! Given the number of empty systems targeted by Thargoids which are in the wrong direction from us, it appears as if shaping the advance is quite possible. In that case, one wonders whether the best strategy actually is to save defensible systems more quickly to keep the battle there deliberately... though this is also likely to occur naturally as well.